Transaction management in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a transaction in database management?

A transaction is a group of one or more operations performed as a single unit. It ensures that either all operations succeed together or none do, keeping data safe and consistent.

intermediate

What are the four key properties of a transaction (ACID)?

ACID stands for:
1. Atomicity - all or nothing
2. Consistency - data stays correct
3. Isolation - transactions don’t interfere
4. Durability - changes are saved permanently

beginner

How do you start a transaction in PHP using PDO?

Use the method beginTransaction() on your PDO object to start a transaction.
Example:
$pdo->beginTransaction();

beginner

What PHP PDO methods are used to save or cancel a transaction?

To save changes, use commit().
To cancel changes, use rollBack().
Example:
$pdo->commit(); or $pdo->rollBack();

beginner

Why is transaction management important in web applications?

It keeps data safe when multiple users or processes work at the same time. It prevents errors like partial updates or lost data, making apps reliable and trustworthy.

Which method starts a transaction in PHP PDO?

A initTransaction()

B start()

C openTransaction()

D beginTransaction()

The correct method to start a transaction in PHP PDO is beginTransaction().

What does the commit() method do in transaction management?

A Saves all changes made during the transaction

B Starts a new transaction

C Cancels the transaction

D Checks for errors

commit() saves all changes made during the transaction permanently.

Which ACID property ensures that transactions do not affect each other?

A Atomicity

B Durability

C Isolation

D Consistency

Isolation ensures that transactions run independently without interfering with each other.

If an error occurs during a transaction, which method should be called to undo changes?

A rollBack()

B commit()

C beginTransaction()

D save()

The rollBack() method undoes all changes made during the transaction.

Why is transaction management important in multi-user applications?

A To speed up the application

B To prevent data conflicts and keep data consistent

C To reduce server load

D To allow unlimited users

Transaction management prevents conflicts and keeps data consistent when many users access the system.

Explain the ACID properties of a transaction and why each is important.

Think about how each property helps keep data safe and reliable.

Describe how to use PHP PDO to manage a transaction including starting, committing, and rolling back.

Imagine you are making changes to a database and want to control when they are saved or undone.

Practice

1. What is the main purpose of using beginTransaction() in PHP when working with databases?

easy

A. To start a group of database operations that can be committed or rolled back together

B. To immediately save changes to the database

C. To close the database connection

D. To check if the database is available

Solution

1. Step 1: Understand the role of beginTransaction()

   This function starts a transaction, grouping multiple database changes into one unit.

2. Step 2: Recognize transaction benefits

   It allows committing all changes at once or rolling back if something goes wrong, ensuring data integrity.

3. Final Answer:

   To start a group of database operations that can be committed or rolled back together -> Option A

4. Quick Check:

   beginTransaction() starts a transaction [OK]

Hint: beginTransaction() starts safe grouped DB changes [OK]

Common Mistakes:

• Thinking it saves changes immediately
• Confusing it with closing the connection
• Assuming it checks database availability

2. Which of the following is the correct syntax to commit a transaction in PHP PDO?

easy

A. $pdo->commitTransaction();

B. $pdo->transactionCommit();

C. commit($pdo);

D. $pdo->commit();

Solution

1. Step 1: Recall PDO commit method

   The correct method to save all changes in a transaction is commit() called on the PDO object.

2. Step 2: Check method names

   Only $pdo->commit(); matches the correct PDO syntax; others are invalid method names.

3. Final Answer:

   $pdo->commit(); -> Option D

4. Quick Check:

   commit() saves transaction changes [OK]

Hint: Use $pdo->commit() to save transaction changes [OK]

Common Mistakes:

• Using incorrect method names like commitTransaction()
• Trying to call commit as a function with argument
• Confusing commit with rollback

3. What will be the output of the following PHP code snippet?

$pdo->beginTransaction();
$pdo->exec("INSERT INTO users (name) VALUES ('Alice')");
$pdo->rollBack();
echo $pdo->query("SELECT COUNT(*) FROM users")->fetchColumn();

medium

A. 0

B. 1

C. SyntaxError

D. Throws Exception

Solution

1. Step 1: Analyze transaction commands

   The code starts a transaction, inserts a user, then rolls back, undoing the insert.

2. Step 2: Check final data count

   Since the insert was rolled back, the users table remains unchanged, so count is 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   rollBack() undoes insert, count = 0 [OK]

Hint: rollBack() cancels changes, so count stays zero [OK]

Common Mistakes:

• Assuming insert stays after rollBack()
• Confusing commit() with rollBack()
• Expecting syntax or runtime error

4. Identify the error in this PHP transaction code:

try {
  $pdo->beginTransaction();
  $pdo->exec("UPDATE accounts SET balance = balance - 100 WHERE id = 1");
  $pdo->exec("UPDATE accounts SET balance = balance + 100 WHERE id = 2");
  $pdo->commit();
} catch (Exception $e) {
  $pdo->commit();
}

medium

A. Missing beginTransaction() call

B. Using commit() in catch block instead of rollBack()

C. No exception handling block

D. Using exec() instead of query()

Solution

1. Step 1: Review transaction error handling

   In case of error, changes should be undone with rollBack(), not committed.

2. Step 2: Identify catch block mistake

   The catch block incorrectly calls commit(), which saves partial or faulty changes.

3. Final Answer:

   Using commit() in catch block instead of rollBack() -> Option B

4. Quick Check:

   Errors need rollBack(), not commit() [OK]

Hint: Use rollBack() on error, not commit() [OK]

Common Mistakes:

• Calling commit() in catch block
• Forgetting to handle exceptions
• Confusing exec() and query() methods

5. You want to transfer money between two accounts safely using PHP transactions. Which of the following sequences of steps is correct?

hard

A. Begin transaction, update sender balance, commit transaction, update receiver balance

B. Update sender balance, update receiver balance, begin transaction, commit transaction

C. Begin transaction, update sender balance, update receiver balance, commit transaction

D. Update sender balance, begin transaction, update receiver balance, commit transaction

Solution

1. Step 1: Understand transaction grouping

   All related updates must be inside the transaction to ensure atomicity.

2. Step 2: Check step order

   Starting with beginTransaction(), then updating both accounts, and finally committing ensures either all changes apply or none.

3. Final Answer:

   Begin transaction, update sender balance, update receiver balance, commit transaction -> Option C

4. Quick Check:

   All DB changes inside transaction before commit [OK]

Hint: Start transaction before updates, commit after all done [OK]

Common Mistakes:

• Updating before starting transaction
• Committing too early before all updates
• Not grouping all changes inside transaction